With the ever-increasing demand for portable communication devices, reliability and efficiency of both user devices and devices in the supporting network has become of increasing importance. There are a number of different considerations, related to both individual elements as well as system elements, which affect these characteristics. For example, it is desirable to increase both bandwidth efficiency and power efficiency of a power amplifier in various communication devices. While bandwidth efficiency (the rate that data can be transmitted over a given bandwidth) is typically achieved using linear modulation, amplifier efficiency is a significant concern for achieving longer battery life and lower energy costs in transmitters as it usually dominates the power consumption in the system.
A transmitter of the portable communication device or of a communication device in the infrastructure (such as a base station) generally uses a radio frequency power amplifier (RFPA) as the final amplifying stage of a transmitter. The RFPA typically has a fixed power supply voltage. With a fixed supply voltage, however, the efficiency of the RFPA decreases as the output signal magnitude drops, leading to ineffectiveness and excessive peak power capability. To improve efficiency, it is desirable for the RFPA to continually operate near saturation, where the amplifier is close to or slightly gain compressed (about 0.5 dB below gain compression to 0.5 dB in gain compression). This can be achieved by modulating the power supply of the RFPA using the known technique of envelope tracking (i.e. the supply voltage of the RFPA tracks the output signal of the RFPA), which adjusts the power supply of the RFPA such that the power supply voltage of the RFPA follows the output signal thereby allowing the RFPA to continually operate near saturation.
While it is usually desirable in using power supply modulation for the RFPA to continually operate near saturation, this may be undesirable when certain unexpected situations arise within the circuitry of the communication device. These conditions, hereinafter referred to as alarm conditions or merely alarms, include significant thermal increases (one type of impairment) or the failure of one or more transistors or other components within the power amplifier module. Under circumstances such as the above impairment or failure, a significant degradation in adjacent channel power (ACP) or other similar issues that cause the power amplifier to operate at a reduced performance (below desired specification) occur if the modulated supply were to operate at a level determined before the impairment or failure. An impairment or failure may also cause the communication device to fail regulatory requirements set by the FCC. Accordingly, when an alarm occurs the output power is thus reduced to avoid such problems.
However, if a false alarm occurs, causing a reduction in the output power, the power supply modulator will converge to this lower output power in order to maintain operation near saturation. Although the RFPA cuts back in power, the supply voltage level should remain at its current level. When the alarm is cleared, the RFPA will attempt to be set back to full power. Unfortunately, an insufficient amount of time exists to return to the power supply to its optimal level before a significant degradation in ACP occurs if the supply modulator has converged to maintain operation near saturation during a false alarm.
Therefore, a need exists for improved control of modulation of a power supply voltage to a power amplifier in order for the power amplifier to maintain high efficiency and that allows the power supply modulator implementation to adapt to various non-ideal conditions that may arise in the communication device.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of the embodiments of shown.
The apparatus and method components have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments shown so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein. Other elements, such as those known to one of skill in the art, may thus be present.